I had my first interview at a national career fair. It was 1 on 1, average difficulty -- walk me through your resume, 2 behavioral questions. They followed up about 6 weeks later with a secondary phone interview. I definitely over-prepared; the interview was very casual (walk me through the software you know, are you comfortable relocating, what classes were your favorite) and was with the lead engineer whose department the intern position would be working under.

Panel interview over the phone. Each sat in different location, but from the same department. The overall process took almost 2 months. Lots of behavioral questions and questions asking how have I dealt with similar situations in the past.